This invention relates to the measurement of material properties and is more particularly directed to the nondestructive testing of substrates for microwave circuits to determine their dielectric constant.
Precise knowledge of the dielectric constant and loss tangent of ceramic substrates is essential to the design of microstrip and integrated microwave circuits. Accordingly, accurate measurement methods are required by the circuit designer and by the substrate manufacturer as well. For both, the simplicity and convenience of the method of measuring are important, but the user and producer may judge the merits of these measurements differently.
Methods proposed previously require intimate contact between the dielectric and some conducting circuit electrode. Such contact may be achieved either by metallization or by pressing a circuit formed on a soft substrate against the ceramic. With metallization of two or all six sides of the substrate to form a cavity resonator, the dielectric constant and loss tangent can be calculated from frequency and quality factor (Q) data. Other procedures, also dependent on metallization, derive their results from measurements on specially fabricated circuits or from circuits such as microstrip lines that may be provided by the manufacturer.
These methods are a compromise between convenience and accuracy. In the methods that require metallization, convenience for the user depends largely on the metallization pattern supplied by the manufacturer. For the producer of unmetallized substrates, metallization is a costly and inconvenient requirement. Since this metallization is an irreversible process that wastes substrates, it is suited only for sampling a production run.
The method involving pressure contact with a circuit on a soft substrate is more suited for production-run testing, although it requires calibration with a standard substrate.